#include<iostream>
#include<algorithm>
using namespace std;

void quick_sort(int a[], int len) {
    // 写快排
}

int main () {
    int m, s, c, a[200] = {0}, cha[200]={0};
    bool v[200] = {false};
    
	cin >> m >> s >> c;
    for (int i=0; i<c; i++) {
        cin >> a[i];
    }

    sort(a, a + c);

    int ans = c;

    for (int i=0; i<c-1; i++) {
        cha[i] = a[i+1] - a[i] - 1;
    }

    sort(cha, cha + c - 1);

    for (int i=0; i<c-m; i++) {
        ans += cha[i];
    }

    cout << ans;
    return 0;
}
